1. Field of the Invention
This invention relates to a method for forming a nitridized interface on a substrate, and more particularly to improving uniformity of an ammonia plasma surface treatment on a semiconductor substrate. Specifically, this invention relates to forming a nitridized polysilicon interface using ammonia plasma doped with fluorine.
2. Description of Related Art
Silicon nitride films are commonly employed in the fabrication of circuits for use in modern semiconductor devices, such as the fabrication of metal-oxide semiconductor (xe2x80x9cMOSxe2x80x9d) devices for high density integrated circuits and submicron designs. For example, silicon nitride films are employed in the manufacture of MOS devices using Local Oxidation of Silicon processes (xe2x80x9cLOCOSxe2x80x9d), as well as advanced LOCOS methods such as polysilicon buffered LOCOS processes (xe2x80x9cPBLOCOSxe2x80x9d). Further information on LOCOS-based processing technology may be found in Wolf.
In a LOCOS manufacturing method, a pad of native oxide is typically formed on a semiconductor substrate for purposes of cushioning transition of stress between a silicon substrate and a silicon nitride film, which is deposited to serve as an oxidation mask. Such pad oxides may be thermally-grown or deposited using chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). In a PBLOCOS method, a thin pad layer of thermally deposited silicon dioxide (SiO2) in combination with a polysilicon buffer layer is formed. PBLOCOS methods are typically utilized to enhance suppression of lateral oxidation and to provide a stress buffer layer between the oxide layer and a subsequently deposited silicon nitride layer.
In a typical LOCOS-based process, silicon nitride and polysilicon buffer layers (when present) are removed selectively to expose those areas where field oxide growth is desired, leaving the active areas of the device covered. Field oxide is then grown xe2x80x9clocallyxe2x80x9d in the etched areas between the active areas covered by silicon nitride film to isolate them from each other. It is desirable to minimize the space required for these isolation zones, as they consume valuable semiconductor space.
It is at the time of field oxide growth that encroachment of oxide into the silicon substrate and interface area between the surrounding silicon nitride and polysilicon buffer layer (if present) typically occurs. This oxide encroachment is commonly referred to as a xe2x80x9cfirst bird""s beakxe2x80x9d when it extends into the substrate, and as a xe2x80x9csecond bird""s beakxe2x80x9d when it occurs in the interface of the silicon nitride and polysilicon. Bird""s beak formation can change the active area size and potentially cause gate poly bridging. First bird""s beak areas consume additional space as they extend beyond the edges of the isolation zones, and work against the achievement of isolation requirements for submicron devices. Second bird""s beak areas tend to interfere with removal of polysilicon in a LOCOS-based process, leaving undesirable xe2x80x9cstringersxe2x80x9d of poly which may cause shorts and/or leakage. Following field oxide growth, the remainder of the silicon nitride layer and any buffer layer present underneath is typically removed in order that the active areas of the semiconductor device may be formed.
Substrate surfaces are typically nitridized using a CVD process such as ammonia-plasma CVD. During surface nitridization in a plasma enhanced CVD reactor, flow of reactant gasses, as well as reacted byproducts, typically results in nonuniform film thickness. Nonuniformity of treatment may be gauged or measured in terms of both film within wafer thickness nonuniformity, and in wafer to wafer nonuniformity. Film within wafer thickness nonuniformity may be expresses in terms of percentage standard deviation. Wafer to wafer nonuniformity typically averages from about 6 xc3x85 to about 10 xc3x85.
Increased film nonuniformity typically results in increased second bird""s beak formation. Such variations in film treatment tend to decrease wafer yield by increasing the second bird""s beak size, leading to poly stringer formation and reduced process capability as measured on a test wafer run with the product. Process capability is defined as the process spec width divided by (6xcex5), and is typically expressed as Cp or Cpk.
As an example of problems encountered with conventional LOCOS-based processes, FIG. 1 illustrates field oxide 18 formed on a semiconductor substrate 10 in isolation area 11, using a conventional LOCOS-based process known in the art. As shown in FIG. 1, isolation area 11 is defined between active device areas 13. Active device areas 13 are covered by silicon dioxide pad layer 12, polysilicon buffer layer 14 and silicon nitride layer 16. As may be seen in FIG. 1, field oxide 18 extends into active areas 13 due to encroachment of oxide 18 into the substrate 10, forming xe2x80x9cfirst bird""s beakxe2x80x9d areas 17, and between silicon nitride layer 16 and polysilicon buffer layer 14, forming xe2x80x9csecond bird""s beakxe2x80x9d areas 19. Upon subsequent removal of layers 16, 14 and 12, residual areas of polysilicon buffer layer 14 may remain between the xe2x80x9cbird""s beakxe2x80x9d areas 17 and 19, which tends to act as a mask against removal of polysilicon. These residual polysilicon areas are referred to as xe2x80x9cpoly stringersxe2x80x9d and are undesirable due to their tendency to cause shorts and leakage.
Using the disclosed method, a fluorine doped nitride surface treatment may be employed to form a fluorine-doped nitridized substrate interface. Benefits of the disclosed method include, but are not limited to, reduction in nonuniformity of nitridized substrate surfaces and wafer to wafer nonuniformity, as well as the provision of a more stable process. By increasing uniformity of substrate nitridization, wafer yield may be increased over conventional undoped ammonia plasma treatment processes by, for example, substantially eliminating across-wafer nonuniformity and the presence of a xe2x80x9csecond bird""s beakxe2x80x9d in the field oxide edge areas formed in a LOCOS-based process. Advantageously, in one embodiment of the disclosed method, process capability (Cp, Cpk) is increased because of this reduction in thickness variation, and/or formation of poly stringers is suppressed or substantially prevented.
While not wishing to be bound by theory, it is believed that lateral oxidation of the poly/nitride interface during field oxidation is prevented by tying up available Si atoms at the surface of the poly. This is believed to result due to the breaking of stressed Sixe2x80x94Oxe2x80x94Si bonds at the polysilicon surface by HF formation and surface reaction, direct F reaction, and/or bombardment. When this occurs, Sixe2x80x94Oxe2x80x94Si bonds are believed to be replaced by Sixe2x80x94F, non-bridging Sixe2x80x94O, and/or Sixe2x80x94N bonds. Formation of Sixe2x80x94N bonds is believed to be the predominate reaction, and it is believed that these bonds create a more uniform interface between a polysilicon layer and a subsequent silicon nitride layer. Furthermore, fluorine is also believed to break Sixe2x80x94H bonds and Sixe2x80x94OH bonds which may form in NH3 plasma, thus tending to form stronger Sixe2x80x94F bonds. It is believed that one or more of the previously described mechanisms retard lateral diffusion and reaction of oxygen during field oxide growth, thus yielding better uniformity.
In one embodiment of the disclosed method, a source of fluorine, typically carbon hexafluorine, C2F6 (Halocarbon-116), is introduced into an ammonia plasma to nitridize a substrate surface (such as oxide or polysilicon) in a LOCOS-based process, for example, a LOCOS or PBLOCOS isolation scheme. LOCOS-based processes are known in the art and are described, for example, in Wolf, Stanley Silicon Processing for the VLSI Era, Volume 2xe2x80x94Process Integration, Lattice Press, Sunset Beach, Calif., pp. 12-41, 1990, which is incorporated herein by reference.
In one embodiment, C2F6 gas (typically employed as an etchant) is introduced into an ammonia plasma during nitride surface treatment of polysilicon. In this embodiment, addition of fluorine-based dopant may surprisingly be used to reduce film within wafer thickness nonuniformity to less than or equal to about 2 xc3x85 (or about 1"sgr"), as measured on a test wafer, resulting in reduction in wafer to wafer nonuniformity and providing a more stable process.
Advantages of the disclosed method may be realized in any semiconductor fabrication surface treatment process employing silicon nitride. For example, the disclosed method may be employed in the manufacture of MOS semiconductor devices on a silicon substrate including, but not limited to, manufacture of DRAM devices.
In one respect, disclosed is a method of nitridizing the surface of a semiconductor substrate, including forming a fluorine-doped nitridized surface on an upper surface of the substrate, such that an interface is defined between the fluorine-doped nitridized surface and the substrate surface. The forming may include exposing the substrate surface to a treatment gas including a fluorine component and a nitrogen component; such that the exposure results in the formation of a fluorine-doped nitridized surface having an interface with the substrate surface. The substrate surface may include silicon dioxide or polysilicon. In one embodiment, the nitrogen component may be at least one of ammonia, nitrogen, or a mixture thereof; the fluorine component may be at least one of C2H6, C3F8, CF4, or a mixture thereof; and the exposing may occur in a chemical vapor deposition process. In another embodiment, the nitrogen component may be ammonia; the fluorine component may be carbon hexafluorine; and the exposing may occur in a low pressure plasma enhanced chemical vapor deposition process. The fluorine-doped nitridized surface may include a film may have a thickness of at least about 1 xc3x85 to about is 40 xc3x85, alternatively from about 10 xc3x85 to about 30 xc3x85. The fluorine-doped nitridized surface may be formed using a treatment gas including a volume ratio of carbonhexafluorine to ammonia of from about 1:1 to about 1:20.
The method may further include exposing the fluorine-doped nitridized surface to an undoped treatment gas including a nitrogen component; such that the exposure results in the formation of an undoped nitridized surface on the fluorine-doped nitridized surface. In such a case, the nitrogen component of the fluorine doped treatment gas may be ammonia; the fluorine component of the fluorine-doped treatment gas may be carbon hexafluorine; the nitrogen component of the undoped treatment gas may be ammonia; and the exposing to the fluorine-doped treatment gas and the exposing to the undoped treatment gas each may occur in a respective low pressure plasma enhanced chemical vapor deposition process. The fluorine-doped nitridized surface may include a film may have a thickness of from about 1 xc3x85 to about 40 xc3x85; and the undoped nitridized surface may include a film may have a thickness of from about 700 xc3x85 to about 3000 xc3x85. The substrate may include a semiconductor wafer, and an upper surface of the undoped nitridized surface may have a within wafer thickness variation of less than about 2 xc3x85.
In another respect, disclosed are MOS semiconductor devices (such as DRAM memory devices), and methods for forming localized field oxide during fabrication of such devices on a substrate having a silicon dioxide pad layer present on an upper surface of the substrate and a polysilicon buffer layer disposed on an upper surface of the silicon dioxide pad layer, including forming a fluorine-doped nitridized surface on an upper surface of the polysilicon buffer layer; such that an interface is defined between the fluorine-doped nitridized surface and the upper surface of the polysilicon buffer layer; forming an undoped nitridized surface on the fluorine-doped nitridized surface, the undoped nitridized surface and the fluorine-doped nitridized surface together forming a silicon nitride layer; defining at least one active region pattern on the silicon nitride layer; removing the silicon nitride and the polysilicon buffer layers in an area outside the active region pattern; and forming a field oxide region in the area where the silicon nitride layer and polysilicon buffer layers have been removed. The formation of a fluorine-doped nitridized surface may include exposing the upper surface of the polysilicon buffer layer to a treatment gas including a mixture of carbon hexafluorine and ammonia in a low pressure plasma enhanced chemical vapor deposition process. The formation of an undoped nitridized surface may include exposing the fluorine-doped nitridized surface to a treatment gas including ammonia and substantially no fluorine component in a low pressure plasma enhanced chemical vapor deposition process. The fluorine-doped nitridized surface may include a film may have a thickness of at least about 10 xc3x85 to about 30 xc3x85, alternatively from about 20 xc3x85 to about 25 xc3x85. The undoped nitridized surface may have a thickness of from about 700 xc3x85 to about 3000 xc3x85, alternatively from about 1200 xc3x85 to about 1800 xc3x85. The fluorine-doped nitridized surface may be formed using a treatment gas including a volume ratio of carbonhexafluorine to ammonia of from about 1:3 to about 1:20. The substrate may include a semiconductor wafer, and an upper surface of the undoped silicon nitride layer may have a within wafer thickness variation of less than about 2 xc3x85. In one embodiment, encroachment of oxide into the interface defined between the fluorine-doped nitridized surface and the upper surface of the polysilicon buffer layer may be inhibited or substantially prevented during formation of the field oxide region.
In another respect, disclosed are MOS semiconductor devices (such as DRAM memory devices), and methods for forming localized field oxide during fabrication of such devices on a silicon substrate, including forming a pad layer of silicon dioxide on the silicon substrate; forming a buffer layer of polysilicon on the silicon dioxide pad layer, the pad layer of silicon dioxide being disposed between the buffer layer of polysilicon and the silicon substrate; forming a fluorine-doped nitridized surface on an upper surface of the polysilicon buffer layer; wherein an interface may be defined between the fluorine-doped nitridized surface and the upper surface of the polysilicon buffer layer; forming an undoped nitridized surface on the fluorine-doped nitridized surface, the undoped nitridized surface and the fluorine-doped nitridized surface together forming a silicon nitride layer; defining at least one active region pattern on the silicon nitride layer, removing the silicon nitride layer and the polysilicon buffer layer in the area outside the active region pattern; and forming a field oxide region on the silicon substrate in the area where the silicon nitride layer and polysilicon buffer layers have been removed. Formation of a fluorine-doped nitridized surface may include exposing the upper surface of the polysilicon buffer layer to a treatment gas including a mixture of carbon hexafluorine and ammonia in a low pressure plasma enhanced chemical vapor deposition process; and formation of an undoped nitridized surface may include exposing the fluorine-doped nitridized surface to a treatment gas including ammonia and substantially no fluorine component in a low pressure plasma enhanced chemical vapor deposition process. In one embodiment, a fluorine-doped nitridized surface-may include a film may have a thickness of from about 20 xc3x85 to about 25 xc3x85, and/or an undoped nitridized surface may have a thickness of from about 1200 xc3x85 to about 1800 xc3x85. The fluorine-doped nitridized surface may be formed using a treatment gas including a volume ratio of carbonhexafluorine to ammonia of from about 1:3 to about 1:15. Using one embodiment of this method, an upper surface of the silicon nitride layer surface may have a within wafer thickness variation of less than about 2 xc3x85. In another embodiment, encroachment of oxide into the interface defined between the fluorine-doped nitridized surface and the upper surface of the polysilicon buffer layer may be inhibited or substantially prevented during the formation of the field oxide region.